Pump lasers are generally found in the form of Fabry-Perot (FP) cavity lasers whose multimode spectrums are broadband and extremely sensitive to temperature and laser drive current and, therefore, requires stabilization in most applications such as optical amplifiers. Different methods have been proposed in the past to stabilize a single laser or a system of multiple lasers.
In a first method, a pump laser is coupled at its output to a reflection filter, such as Fiber Bragg Gratings (FBG), which selectively reflects part of the laser spectrum back towards the laser and, therefore, stabilizes the laser spectrum and power. This first method has been extensively used to stabilize a single laser. Some multi-wavelength applications have also used the first method to stabilize multiple lasers using individual FBGs followed by a wavelength division multiplexer (WDM) to combine the stabilized laser signals.
Referring now to FIG. 1, there is shown a schematic of a prior art arrangement 10 for stabilizing a single laser source 11 using a combination of a transmission filter 12 having a wavelength spectral response of F(w) and a reflector 13 that are shown within a dashed line rectangle illustrating the second method indicated hereinabove. In the arrangement 10, an output/input facet 11a of the laser source 11 is coupled to a first input/output port 12a of the transmission filter 12 via a path A. A second output/input port 12b of the transmission filter 12 is coupled to a first input/output port 13a of the reflector 13 via a path B. A second port 13b of the reflector 13 provides an output signal from the stabilized laser system arrangement 10 via a path C.
The transmission filter 12 sets the wavelength based on its spectral response F(w), and the reflector 13 sets the amount of signal reflection provided back through the transmission filter 12 to the laser source 11. When a portion of the signal filtered by the transmission filter 12 is reflected by the reflector 13, it is again filtered by the transmission filter 12 with the spectral response F(w) to provide a feedback signal to the output facet 11a of the laser source 11. In response to the feedback signal, it is found that the laser source 11 produces a wavelength shift δw in a first direction, and generates an output signal that now peaks at a center wavelength that is shifted by an amount δw from the peak wavelength of transmission filter F(w) and is no longer at the desired wavelength as is shown in FIG. 2, resulting in excess loss and system inefficiency.
Referring now to FIG. 2, there is shown a graph of wavelength (w) on the X-axis versus Intensity (dB) on the Y-axis for exemplary curves 16 and 17 representing the forward filter spectral response and the feedback filter spectral response, respectively, and an exemplary output signal center wavelength represented by the line 18. In operation, the output signal (not shown in FIG. 2) generated by the laser source 11 is filtered once by the transmission filter 12 with a spectral response curve F(w) as is shown by curve 16 to provide an output signal from the system 10 where the power peaks at a center wavelength represented by the line 18. For the system 10 of FIG. 1, the forward filter spectral response for the transmission filter 12 is defined as Fo(w)=F(w) as the transmission filter spectral response between the output/input facet 11a of the laser source 11 and the output port 13b from the reflector 13.
When a portion of this filtered signal is reflected by reflector 13, it is again filtered for the second time by the transmission filter 12 resulting in a narrower signal and returned to the output/input facet 11a of the laser source 11. The feedback filter spectral response for the system 10 is defined as the spectral response between the forward and backward (feedback) signals found at the output/input facet 11a of the laser source 11, Ff(w)=F(w)f·F(w). As a result of the fed back signal, it is found that the laser source 11 produces a wavelength shift and now generates an output signal that now peaks at the center frequency shown by line 18 which is separated by an amount δw from the peak of curve 16 as is shown in FIG. 2. This results in an excess loss of power in the output signal of the system 10. The above description indicates that the laser source 11 produces a red shift (e.g., a first direction) in response to a feedback signal. The occurrence of a red shift (in the first direction) shown in FIG. 2 is mostly true for semiconductor diodes lasers. However, there are other types of lasers that actually produce a blue shift (in a second opposite direction from a red shift) in response to the reception of a feedback signal that also causes a similar excess loss.
The main requirements for pump sources in optical amplification are power efficiency, relative intensity noise (RIN), stimulated Brillouin Scattering (SBS), and spectral stability. It is desirable to provide a stabilized multi-wavelength source that provides for the requirements of power efficiency, stimulated Brillouin Scattering (SBS), and spectral stability.